1. Field of the Invention
The present invention relates to a technology for forming a less color-shifted image.
2. Description of the Related Art
Existing color-image forming apparatuses in general are equipped with a plurality of photosensitive members and an optical scanner that includes a polygon mirror as an optical deflector to scan each photosensitive member. However, due to heat generated while the optical scanning device etc. is in operation, temperature inside the device increases as the time passes and each unit undergoes thermal expansion. Consequently, scanning beams that scan the photosensitive members may shift in a sub-scanning direction, and therefore the image quality deteriorates.
To overcome the above problem, a controlling method is disclosed in Japanese Patent Application Laid-open No. H3-293679 and Japanese Patent Application Laid-open No. H9-244332, by which temperature inside the apparatus is detected, image formation timing is corrected based on detection results, and the position of the image formed on the photosensitive members in a predetermined time is corrected.
As disclosed in Japanese Patent Application Laid-open No. 2004-246010, Japanese Patent Application Laid-open No. 2004-271548, and Japanese Patent Application Laid-open No. 2003-322817, the optical scanning device includes an airflow path, a fan, and a radiation fin, and prevents deterioration of image quality by suppressing the increase in temperature inside the optical scanning device.
However, in the technology disclosed in Japanese Patent Application Laid-open No. H3-293679 and Japanese Patent Application Laid-open No. H9-244332, the image-formation correction control makes the image forming apparatus complicated. Furthermore, in the technology disclosed in Japanese Patent Application Laid-open No. 2004-246010, Japanese Patent Application Laid-open No. 2004-271548, and Japanese Patent Application Laid-open No. 2003-322817, the increase in temperature inside the optical scanning device is controlled by installing fan etc. However, in recent times, for energy conservation and noise reduction, rotation speed of the fan inside the image forming apparatus is reduced or stopped in standby mode. Consequently, cooling efficiency drops considerably and usually it becomes difficult to cope up with the image quality deterioration.
In present conditions, laser printers and digital copying machines are required to have high quality image, high speed, occupy less space, energy conservation, low cost etc. Particularly, high image quality is essential for the color-image forming apparatus, and therefore it is important to deal with the problems regarding color alignment.